1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and, more particularly, to an image forming apparatus employing an electro-photographic image forming method, and an image forming unit included in the image forming apparatus.
2. Description of the Related Art
Generally, an image forming apparatus is an apparatus to develop a black-and-white image or color image on a printing medium, such as a sheet of paper, on the basis of an image signal. Examples of image forming apparatuses include a laser printer, an ink-jet printer, a copying machine, a multi-function machine, a facsimile device, etc. Representative image forming methods of these various kinds of image forming apparatuses include an electro-photographic method and an ink-jet method. In the electro-photographic method, a light beam is scanned onto a photosensitive member so as to form an electrostatic latent image, and a toner is attached onto the electrostatic latent image to transfer the electrostatic latent image onto a printing medium. In the ink-jet method, liquid-phase ink is injected onto a surface of a printing medium in response to an image signal.
In an electro-photographic image forming apparatus, more specifically, after the surface of a photosensitive member is electrically charged with a predetermined electric potential, a light beam is scanned onto the photosensitive member so as to form an electrostatic latent image by use of a potential difference, and subsequently, a toner as a developing agent is attached onto the electrostatic latent image, to form a visible image. Then, the visible image formed on the photosensitive member is transferred onto a printing medium, and thereafter, heat and pressure are applied to the printing medium so as to fix the visible image, formed of powder developing agent, onto the surface of the printing medium.
Electro-photographic developing methods may be classified into a unary developing agent method using an insulating toner or conductive toner, and a binary developing agent method using a toner and a magnetic carrier. In the unary developing agent method, toner particles are electrically charged by friction between the respective toner particles or between the toner particles and a charged member, and thereafter, are conveyed onto a photosensitive member by a developing unit. In the binary developing agent method, magnetic carrier particles and non-magnetic toner particles made of a synthetic resin are mixed with each other by an appropriate mixing ratio, and the toner particles are electrically charged while being mixed with the carrier particles. Thereby, after the toner particles are conveyed, along with the carrier particles, onto a magnetic roller, the toner particles are transferred onto a photosensitive member.
Of the above described developing methods, an image forming unit which uses the unary developing agent method includes a developing unit to attach a toner to a photosensitive member formed with an electrostatic latent image, a regulating blade to regulate the thickness of the toner attached to the surface of the photosensitive member, and a supply unit to supply the toner to the developing unit.
The developing unit includes a metallic roller shaft and a rubber conductive layer coupled to an outer surface of the roller shaft, and is installed to come into contact with or to be separate from the photosensitive member. The toner, attached to the developing unit, is conveyed onto the photosensitive member by an electric field generated between the developing unit and the photosensitive member. The regulating blade serves to achieve an even thickness of the toner layer attached to the surface of the developing unit and also, serves to electrically charge the toner attached to the developing unit. The supply unit is made of sponge or urethane foam. The supply unit serves to remove the excess toner, which still remains on the developing unit without being conveyed onto the photosensitive member, while being rotated in contact with the developing unit. In addition, the supply unit serves to attach new toner to the developing unit and also, to frictionally charge the toner attached to the developing unit. Predetermined voltages are applied to the respective developing unit, the regulating blade, and the supply unit.
However, since the volume resistance of the developing unit is changed according to a change of temperature and humidity, the above described image forming unit may cause an unwanted change in the quality of an image when environmental conditions are changed. In particular, under a low-temperature and low-humidity environment (L/L, for example, 10° C. and 20%), the volume resistance of the developing unit increases, resulting in deterioration of toner recovery ability and printing quality.
To solve the above described problem, there has been proposed a method for lowering the volume resistance of a conductive layer included in a developing unit, so as to reduce a change in the volume resistance of the developing unit according to a change of environment. More specifically, if the volume resistance of the developing unit is less than 10E5 Ωcm, the developing unit experiences a slight change in volume resistance under a low-temperature and low-humidity environment (L/L, for example, 10° C. and 20%), a standard environment (N/N, for example, 23° C. and 50%), and a high-temperature and high-humidity environment (H/H, for example, 32° C. and 80%). Accordingly, it is possible to prevent a change in the quality of an image due to only a slight change of the volume resistance.
However, when the volume resistance of the rubber layer, included in the developing unit, is lowered to less than 10E5 Ωcm, it is impossible to achieve a sufficient electric field between the developing unit and a supply unit due to the problem of electric charge leakage. This causes an insufficient amount of toner to be supplied to the developing unit, resulting in a local density difference of an image and consequently, deterioration in the quality of the image.